Episode 50 is all about Hesperosaurus, one of the oldest stegosaurs from the Morrison Formation.

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In this episode, we discuss:

• The dinosaur of the day:  Hesperosaurus
• Name means “western lizard”
• Lived in the Jurassic
• Fossils first found in 1985, in Wyoming
• Patrick McSherry, a fossil hunter, found the fossils on a ranch in Wyoming. But he had some trouble digging up the bones due to the hard rock, so he got help from Ronald G. Mjos and Jeff Parker from Western Paleontological Laboratories. They also worked with Dee Hall, a paleontologist from Brigham Young University
• At first they thought the bones were Stegosaurus, but Clifford Miles, who prepared the fossils, found they were a new species
• Type species is Hesperosaurus mjosi, named in 2001
• Type species described in 2001 by Kenneth Carpenter, Clifford Miles, and Karen Cloward
• Named “western” because it was found in the western United States. Named “mjosi” to honor Ronald Mjos, who also had a cast made, which you can see in the Denver Museum of Natural History
• In 2001, the holotype was the oldest known stegosaur in America
• Holotype included a nearly complete skull, and most of the body (13 neck vertebrae, 13 back vertebrae, 3 sacrals, 44 tail vertebrae, neck ribs, dorsal ribs, chevrons, left shoulder blade, pelvis, ossified tendons, and 10 neck and back plates (formed in 2 rows)
• Based on healed fractures, the holotype was an old Hesperosaurus individual
• Part of the stegosaur family, but one of the older stegosaurs in the Morrison Formation
• Close relative of Stegosaurus, and like Stegosaurus it had two rows (probably alternating) of plates on its back and four spikes on its tail (thagomizers, informally)
• Informally the tail spikes are called thagomizers, after a Gary Larson “Far Side” cartoon was published in 1982 showing cavemen calling the spikes thagomizers (line was “Now this end is called the thagomizer…after the late Thag Simmons”)
• Plates were not as tall as Stegosaurus, but possibly longer
• Skull was deeper than Stegosaurus
• Kenneth Carpenter originally said Hesperosaurus was a basal stegosaur. But in 2008, Susannah Maidment and colleagues published a study that showed it was a derived form, and a close relative of Stegosaurus and Wuerhosaurus. They said it should be considered to be a species of Stegosaurus (Stegosaurus mjosi instead of Hesperosaurus mjosi); and renamed Wuerhosaurus to Stegosaurus homheni. But this controversial and not everyone agrees (Carpenter and Christiansen both separately rejected this in 2010, saying Hesperosaurus was different enough to be its own genus)
• Carpenter disagreed because he thought that the type species of Stegosaurus, Stegosaurus armatus, because he considered Stegosaurus armatus to be a nomen dubium, and rejected lumping all Stegosaurus fossils from North America into one species. He also thought that S. stenops was a separate species and compared Hesperosaurus to S. stenops. Differences he found include Hesperosaurus having a large antorbital fenestra, a short and deep maxilla, 13 neck vertebrae instead of 10, 13 dorsal (back) vertebrae instead of 17, and oval shaped plates instead of triangular plates
• Published descriptions of Hesperosaurus vary though, due to different interpretations. Plate shape may differ due to erosion, the different numbers of vertebrae may be due to whether some of the vertebrae should be part of the neck or back
• In 2012 an histological study found that Hesperosaurus osteoderms were the same as those in Stegosaurus (CAT-scans showed the plates having thin, dense outer walls, with spongy bone inside, and the spikes had thicker walls and spongy interiors with a single large blood vessel)
• In 2010, a study on the soft visible parts of the “Victoria” specimen found the first proof that a stegosaur had a horn sheath on the plates, which may mean the plates were used for defense (since the horn sheath would have strengthened the plate and given it sharp cutting edges); also reinforces the display theory because the sheath increased the plates’ visible surface, and sheaths are usually brightly colored
• But thermoregulation would have been more difficult, since the horn sheath provides another insulating layer and it has a smooth surface, but modern cattle and ducks use horns and beaks to help cool down, even with their horn sheaths
• Several pretty complete skeletons of Hesperosaurus have been found
• Since 1995 Hans Jacob Siber, a Swiss paleontogist, excavated stegosaur specimens in Wyoming. As of 2001, 3 other Hesperosaurus specimens have been found there (at first they were thought to be Stegosaurus): “Moritz” (found in 1995, a partial skeleton) “Victoria” (also found in 1995, not reclassified until 2010 by Nicolai Christiansen, and has skin and horn sheath impressions as well as a mostly complete skull); “Lilly” (found in 2002 and named after the sisters Nicola and Rabea Lillich who volunteered to help excavate); all three are now part of the Aathal Dinosaur Museum collection in Switzerland
• Not all 3 Swiss specimens have yet been formally described
• Nicolai Christensen and Emanuel Tschopp described “Victoria” (found in the Howe-Stephens quarry, where other soft tissue impressions have been found)
• When Victoria died, she was partially buried, and then completely buried by a second flow of sediment, preserving elements on her right side
• Had enough soft tissue impressions to show it had a honeycomb scale pattern on its skin (also seen in hadrosaurs and one other stegosaur, from Asia, Gigantspinosaurus)
• Scale pattern is not uniform, but is different depending on part of the body (larger, domed scaled are surrounded by smaller ones on its back)
• Concluded there is a plate/horn sheath based on it having a smooth plate with shallow grooves (though not 100% confirmed)
• In 2015, more Hesperosaurus specimens were found (at least 5 in Montana)
• The specimens found in 2015 showed that Hesperosaurus (sometimes referred to as Stegosaurus mjosi) showed sexual dimorphism. Evan Thomas Saitta published a study that showed there were two types of plates
• The paper, “Evidence for sexual dimorphism in the plated dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA,” was published April 22 in PLoS ONE
• Specimens found with two types of plates (large and round v. tall and spiky) were not different species but different gender
• Evan Saitta, lead author said the large wide plates were probably from males (for display), and the tall spiky ones were from females (used as deterrents)
• They were found together, which shows they probably co-existed, and the plates had similar growth rings (so the dinosaurs were around the same age, and it’s not that the plates changed with age)
• Hesperosaurus was 20-23 ft (6-7 m) long and 2-3 tons
• Short, wide head
• Had 20 maxillary teeth on head side (fewer than Steogsaurus), though similar (but larger)
• In 2007 Peter Malcolm Galton wrote that Hesperosaurus teeth were different, due to having rough vertical ridges on the upper part of the crown, and weakly developed fine grooves on the tooth surface
• Herbivore, probably ate moss, cycads, conifers, ferns, horsetails
• Stegosaurs were first around in Asia in the middle to late Jurassic, and then migrated to North America a few million years later
• Predators included Ceratosaurus and Allosaurus
  Some think Hesperosaurus is more closely related to Dacentrurus (a stegosaur from Europe) instead of Stegosaurus
• Stegosauridae is a family that lived in the Jurassic and Cretaceous
• They have rows of osteoderms (plates) and thagomizers on their tails
• May have been used for display, defense, and/or thermoregulation
• Slow, because their front legs were shorter than rear legs, but large and powerful
• Two subfamilies: Dacentrurinae and Stegosaurinae
• Stegosaurinae tend to be larger (earliest one is Lexovisaurus from England)
• Dacentrurinae have long back spines and necks
• Dacentrurinae includes Dacentrurus and stegosaurs more closely related to Dacentrurus than Stegosaurus, such as Hesperosaurus and Miragaia
• Fun Fact: The fastest animal to ever live is technically a dinosaur: the peregrine falcon, which reaches top speeds of 240 mph.